The need for materials for cutting tool applications, exhibiting improved toughness, good strength at elevated temperatures, and chemical inertness, and capable of operating at high cutting speeds has generated a widespread interest in ceramic materials as candidates to fulfill these requirements. Conventional ceramic cutting tool materials have failed to find wide application primarily due to their low fracture toughness.
Therefore, many materials have been evaluated to improve ceramic performance, such as silicon nitride-based composites for cutting tool applications. Specific examples of silicon nitride-based composite cutting tools are discussed in U.S. Pat. No. 4,388,085 to Sarin et al. (composite silicon nitride cutting tools containing particles of TiC); U.S. Pat. No. 4,425,141 to Buljan et al. (a composite modified silicon aluminum oxynitride cutting tool containing particulate refractory metal carbides, nitrides, and carbonitrides); U.S. Pat. No. 4,433,979 to Sarin et al. (composite silicon nitride cutting tools containing particulate hard refractory transition metal carbides or nitrides); U.S. Pat. No. 4,449,989 to Sarin et al. (composite silicon nitride cutting tools coated with two or more adherent layers of refractory materials); and U.S. patent application Ser. Nos. 892,642 and 892,634 both filed Aug. 4, 1986 by Baldoni et al. (composite silicon nitride and silicon aluminum oxynitride materials, respectively, containing refractory transition metal carbide, nitride, or carbonitride whiskers).
Many improvements have been made in the toughness, abrasion resistance, high temperature strength and chemical inertness of such materials, but increased demands by the cutting tool industriy require cutting tools with new and improved characteristics. Titanium diboride has aroused interest because of its hardness, but has heretofore been considered too brittle for use in such applications as cutting tools.
In applications such as gray cast iron machining, ceramic tool wear has been found to be dominated by abrasion. Even at cutting speeds as high as 5000 sfm, chemical reactions between tool and workpiece are negligible in comparison. It has been found that abrasion resistance for, for example, silicon nitride ceramic cutting tool materials is somewhat more dependent on the fracture toughness than the hardness. It may be seen, therefore, that further improvement in the fracture toughness of ceramic materials could bring about significant increases in both reliability and abrasive wear resistance, providing materials for cutting tools with new and improved characteristics. The present invention provides such new and improved ceramic materials.
The wear-resistant titanium diboride-based composites according to the invention are also useful in wear part and structural applications, for example as seals, dies, parts for automotive engines, nozzles, etc, and in impact resistant applications, for example as ceramic armor, etc.